The preset invention relates to a gain control circuit for use in a shading compensation circuit.
In an image reading device applied to a digital copying machine, facsimile machine or the like, a CCD image sensor optically scans a document. Then, the CCD image sensor outputs an image signal for each picture element, each signal corresponding to the image on the document.
The sensitivity of every cell (each cell corresponding to a picture element) of the CCD image sensor is not identical, and even if the CCD image sensor scans a plate having constant density, such as a white reference plate, the amount of incident light on every cell is not equal. Thus, even if the CCD image sensor scans the constant density plate, the level of the image signal output from every cell is not equal. Shading compensation is performed in order to prevent the irregular output of image signals from the CCD image sensor.
During the shading compensation, when the image sensor scans a white reference plate having white constant tone, the image signal output from each cell of the image sensor is controlled so that the level of the image signal from every cell becomes equal. A gain control circuit is used for this control of the image signal.
The gain control circuit includes a digital-to-analog converter (hereinafter termed a D/A converter), for example. The following is a description of a conventional gain control circuit including the D/A converter with reference to FIG. 1.
The image signal from each cell of the image sensor is successively input to a terminal 24. When the image signal V.sub.IN is input to the terminal 24, a input current I.sub.in corresponding to the image signal V.sub.IN is supplied to a terminal (+V.sub.REF) of a D/A converter 21 via a resistor R.sub.1. A terminal (-V.sub.REF) of the D/A converter 21 and the ground are connected by resistor R.sub.2. A digital signal which has 8 bits B.sub.1 (MSB) through B.sub.8 (LSB) is input to the D/A converter 21. This D/A converter 21 outputs a current I.sub.OUT dependent on the value of the digital signal B.sub.1 through B.sub.8 on the basis of the input current I.sub.IN. The output current I.sub.OUT from the D/A converter 21 is input to an inversion input terminal of an operational amplifier 22. D/A converter 21 outputs another current I.sub.OUT. This output current I.sub.OUT is supplied to the ground via a resistor R.sub.3. A non-inversion input terminal of the operational amplifier 22 and the ground are connected by a resistor R.sub.5. In the operational amplifier 22, between the inversion input terminal and an output terminal, a feed back resistor R.sub.4 is connected. The output terminal of the operational amplifier 22 is connected to a terminal 26. The output current I.sub.OUT from the D/A converter 21 is converted into a voltage V.sub.OUT due to a current-voltage conversion function of the operational amplifier 22.
In the gain control circuit having a structure as has been described above, the 8-bit digital signal is a gain control signal V.sub.C. That is, a gain V.sub.IN /V.sub.OUT is controlled according to the value of this gain control signal V.sub.C. Then, when the level of the input signal V.sub.IN is constant, the relationship between the gain control signal V.sub.C and the output signal V.sub.OUT is shown as a solid line as, for example, in FIG. 2. That is, when all bits of the 8-bit gain control signal V.sub.C are height-level (H-level) the gain V.sub.IN /V.sub.OUT is equal to 1, and when all bits thereof are low-level (L-level) the gain V.sub.IN /V.sub.OUT is equal to 1/255. Then, the gain is controlled according to the gain control signal V.sub.C in the 1/255 step between 1/255 and 1.
The image signal output from each cell of the CCD sensor is input to the terminal 24. The gain control signal V.sub.C corresponding to each cell of the CCD image sensor is determined so that when the CCD image sensor scans the white reference plate having constant density, the level of the output signal from the terminal 26 of the gain control circuit corresponding to each cell of the CCD image sensor becomes constant. Then, when the CCD image sensor scans the document, the image signal from each cell of the CCD image sensor is input to the terminal 24, and the 8-bit gain control signal V.sub.C which corresponds to the cell of the CCD image sensor is supplied to the D/A converter 21. Whereby, the output signal V.sub.OUT from the gain control circuit becomes the image signal for which the shading compensation is performed.
The output characteristic of the gain control circuit which is described above is changed due to various factors, as is indicated by the dotted lines in FIG. 2. The output characteristic of the gain control circuit is changed continually so that the level of the output signal V.sub.OUT from the gain control circuit is fluctuated. It is especially difficult to prevent the changing of the output characteristic caused by an offset current of the operational amplifier 22.
To return the changed output characteristic of the gain control circuit to an initial characteristic (see FIG. 2), the gain control circuit which has a structure shown in FIG. 3 has been proposed (see "Linear Data Manual" Vol. 5-111, 112, 121, 122, Nippon Sigunetikkusu Kabushikikaisha, Nov. 14, 1986).
The gain control circuit shown in FIG. 3 has a variable resistor VR.sub.1 which is connected between a+.gradient. power source and a-V power source and a resistor R.sub.6 connected between the output terminal of the D/A converter 21 and a slidable terminal of the variable resistor VR.sub.1. A reference signal having a predetermined level is input to the terminal 24. In this condition, the variable resistor VR.sub.1 is manually adjusted so that the relation between the reference signal (V.sub.IN) and a signal (V.sub.OUT) output from the terminal 26 when the reference signal is input becomes the initial characteristic, as is indicated by the solid line in FIG. 2. The reference signal has the same level as a signal which is output from the image sensor, this reference signal corresponding to a black image having a predetermined tone, for example.
In the gain control circuit shown in FIG. 3, when a drift of the output signal V.sub.OUT occurs due to a fluctuation of environmental temperature or a fluctuation of output voltage of a power source, it is impossible to adjust the drift of the output signal V.sub.OUT so as to cancel it, because the variable resistor VR.sub.1 must be adjusted manually.
If the gain control circuit is structured by using a high-speed and a little drift type D/C converter and the same type operational amplifier, the disadvantage described above does not occur. However, as these circuit elements are very expensive, use of these circuit element leads to increased product cost.